Antigen recognition and response in the mammalian immune system is governed, in part, by the interaction between T-cells and antigen presenting cells. Via its heterodimeric T cell receptor, a T cell recognizes peptide fragments of antigens presented as a complex with major histocompatibility (MHC) molecules (Yewdell and Bennenk, Cell 62: 203, 1990; Davis and Bjorkman, Nature 334: 395, 1988). There are two parallel cellular systems of T cells and antigen presenting molecules which distinguish between two types of antigens, foreign antigens introduced from outside of the cell (such as foreign chemicals, bacteria, and toxins) and endogenous antigens produced within the cell (such as viruses or oncogene products) (Bevan, Nature 325: 192, 1987; Braciale, et al., Immunol. Rev. 98:95, 1987; Germain, Nature 322: 687, 1986).
There are two general classes of MHC molecules, MHC class I, and MHC class II molecules. MHC class I molecules present peptide antigens generally derived from endogenously produced proteins to the CD8+ Tc cells, the predominant cytotoxic T cell that is antigen specific. The MHC class I-related proteolytic system is present in virtually all cells for the purpose of degrading highly abnormal proteins and short-lived molecules or viral proteins. This proteolysis is thought to be non-lysosomal and to involve ATP-dependent covalent conjugation to the polypeptide ubiquitin (Goldberg, et al., Nature 357: 375, 1992). Peptide fragments, possibly in association with a larger proteasome complex, are then postulated to enter into the endoplasmic reticulum or some other type of exocytic compartment (other than the endocytic/lysosomal compartment). There they bind to MHC class I molecules and follow the constitutive secretory pathway from the endoplasmic reticulum through the Golgi to the cell surface where they are presented by the MHC I protein to the CD3-CD8 cytotoxic T cell antigen receptor.
MHC class II molecules generally present antigens that are introduced from outside the cells in a process that involves cellular uptake of molecules comprising the antigens, and generation of antigenic peptide fragments in endosomal/lysosomal organelles. The MHC class II-related process by which foreign antigens are processed in antigen presenting cells (APC) cells is generally believed to occur in an endocytic pathway. Antigens taken into the cell by fluid-phase pinocytosis, absorptive endocytosis, or phagocytosis enter into a late endosomal/lysosomal compartment where large molecules are converted to peptides by digestion through proteases and other hydrolases. During this process, the immunodominant smaller peptides come in contact with and are bound by MHC class II molecules and the peptides are carried to the cell surface. On the cell surface of APC, these short peptides in conjunction with MHC class II molecules bind the CD3-CD4 complex on the surface of helper T cells, activating the replication and immune function of these cells. Following this interaction, helper T cells release lymphokines that stimulate the proliferation and differentiation of leukocytes and inhibit their emigration from the site of infection. In general, the activation of helper T cells by peptide-loaded APC is required for optimal B cell and T cell action, and thus is necessary for proper immune system function.
The exact site of antigen processing and association of processed peptides with MHC class II in the endosomal/lysosomal pathway is as yet unclear. Data have been presented suggesting that MHC class II molecules meet with endocytosed proteins in the early endosomal compartment (Guagliardi, et al., Nature 343: 133, 1990). Partially processed antigens and easily degradable antigens may yield peptides that can combine with MHC class II in the early endosomal compartment. However, evidence is mounting that the major site of antigen processing and association with MHC class II occurs either in the late endosome, the lysosome, or a distinct compartment related to the lysosome (Neefjes, et al., Cell 61: 171, 1990).
The functions of the two types of T cells are significantly different, as implied by their names. Cytotoxic T cells eradicate intracellular pathogens and tumors by direct lysis of cells and by secreting cytokines such as γ interferon. Helper T cells also can lyse cells, but their primary function is to secrete cytokines that promote the activities of B cells (antibody-producing cells) and other T cells and thus they broadly enhance the immune response to foreign antigens, including antibody-mediated and Tc-mediated response mechanisms.
CD4+ T cells are the major helper T cell phenotype in the immune response. Their predominant function is to generate cytokines which regulate essentially all other functions of the immune response. Animals depleted of CD4+ or humans depleted of CD4+ cells (as in patients with AIDS) fail to generate antibody responses, cytotoxic T cell responses, or delayed type hypersensitivity responses. It is well known in the art that helper T cells are critical in regulating immune responses.
CD4+ MHC class II restricted cells have also been shown to have cytotoxic capacity in a number of systems. One of the most important disease-relevant cases in which CD4+ cytotoxic T cells have been demonstrated is in the response to fragments of the HIV gp120 protein (Polydefkis, et al., J. Exp. Med. 171: 875, 1990). CD4+ MHC class II restricted cells also have been shown to be critical in generating systemic immune responses against tumors. In an adoptive transfer model, CD4+ cells are critical in eliminating FBL tumors in mice. In the active immunotherapy model of Golumbek, et al. Science 254: 713, 1991, CD4+ cells have also been shown to be critical in the systemic immune response against a number of different solid malignancies.
Because CD4+MHC class II restricted cells appear to be the critical memory cells in the T cell arm of the immune response, an appropriate vaccination strategy is to generate CD4+ antigen-specific MHC class II-restricted memory T cell populations.
Traditional vaccines rely on whole organisms, either pathogenic strains that have been killed or strains with attenuated pathogenicity. On the one hand, these vaccines run the risk of introducing the disease they are designed to prevent if the attenuation is insufficient or if enough organisms survive the killing step during vaccine preparation. On the other hand, such vaccines have reduced infectivity and are often insufficiently immunogenic, resulting in inadequate protection from the vaccination.
Recently, molecular biological techniques have been used in an attempt to develop new vaccines based on individual antigenic proteins from the pathogenic organisms. Conceptually, use of antigenic peptides rather than whole organisms would avoid pathogenicity while providing a vaccine containing the most immunogenic epitopes. However, it has been found that pure peptides or carbohydrates tend to be weak immunogens, seeming to require a chemical adjuvant in order to be properly processed and efficiently presented to the immune system. A vaccine dependent on T cell responses should contain as many T cell epitopes as would be needed to stimulate immunity in a target population of diverse MHC types. Further, since T cell recognition requires intracellular protein processing, vaccine preparations facilitating internalization and processing of antigen should generate a more effective immune response. Previous attempts to direct antigens to MHC molecules (see, U.S. Pat. No. 4,400,276) were not effective because the antigen-processing step was evaded. A successful hepatitis B vaccine has been prepared using cloned surface antigen of the hepatitis B virus, but this appears to be due to the tendency of the hepatitis surface antigen molecule to aggregate, forming regular particles that are highly immunogenic.
Genetic (DNA) vaccines are new and promising candidates for the development of both prophylactic and therapeutic vaccines. They are proven to be safe and the lack of immune responses to a vector backbone may be a definitive advantage if repetitive cycles of vaccination are required to achieve clinical benefits. However, one potential disadvantage of conventional DNA vaccines is their low immunogenicity in humans. One likely cause of this low immunogenicity is the restricted access of antigens formed within cells to the MHC II pathway for antigen processing and presentation to T helper cells.
U.S. Pat. No. 5,633,234 describe chimeric proteins comprising an antigenic domain and a cytoplasmic endosomal/lysosomal targeting signal which effectively target antigens to that compartment. The antigenic domain was processed and peptides from it presented on the cell surface in association with major histocompatibility (MHC) class II molecules. The cytoplasmic tail of LAMP-1 were used to form the endosomal/lysosomal targeting domain of the chimeric protein.